1. Field of the Invention
This invention relates to a zirconia-based coating composition which can form a transparent, very hard coating film on the surface of metals (e.g. stainless steel, aluminum and the like), inorganic building materials and plastics, can be hardened at low temperatures, can form a thick coating film, can be recoated, and can form a coating film superior in adhesion, corrosion resistance, insulating property, friction resistance, heat resistance, moisture resistance, weather resistance, flame resistance, water resistance, organic chemical resistance and acid resistance and particularly superior in alkali resistance.
2. Description of the Prior Art
In recent years, those coating compositions have been required which can be hardened at low temperatures, can form a thick coating film, can be recoated, and can form a very hard, transparent coating film superior in adhesion, corrosion resistance, insulating property, friction resistance, heat resistance, moisture resistance, weather resistance, flame resistance, water resistance, water impermeability, sea water resistance, organic chemical resistance, acid resistance, alkali resistance, etc.
As coating compositions satisfying only some of the above requirements, there have been proposed an aqueous acidic composition comprising a partial condensate of a silanol and colloidal silica (see U.S. Pat. Nos. 3,986,997 and 4,027,073) and a composition comprising a zirconium alkoxide or the like and a silicon compound having 1-4 alkoxy groups [see Japanese patent application Kokai (Laid-Open) No. 100,943/82].
However, the coating composition capable of forming a transparent coating film as stated in U.S. Pat. Nos. 3,986,997 and 4,027,073 has the following problems: The coating film formed is generally inferior in transparency, alkali resistance, storage stability and adhesion. When the film is as thin as about 20 .mu.m, pin holes tend to be caused in the film. Even after the formulation, the ageing time till the composition becomes usable is long. The coating conditions are narrow. The workability is inferior. The wettability on organic coating film, plastics, etc. is inferior. The composition is difficult to recoat.
Further, the coating composition described in Japanese patent application Kokai (Laid-Open) No. 100,943/82 has the following problems: The composition is unable to form a thick coating film, inferior in storage stability, and liable to cause precipitation. Accordingly, it is difficult to prepare a uniform coating composition.